This invention relates to clutchless type automotive air conditioning compressors, and specifically to a capacity control valve therefor which has a built in by pass feature that is operative during minimum compressor stroke.
Automotive air conditioning compressors, whether of fixed or variable capacity, have typically interposed an electromagnetic clutch between the drive pulley and the compressor drive shaft, which allows the compressor to be entirely disconnected when air conditioning demand is absent or very low. This obviously saves on energy and compressor wear, and prevents evaporator icing that would otherwise occur when cooling demand was low and the compressor continued to pump. With a fixed capacity compressor, a clutch is the only practical way to bring the compressor pumping capacity to zero.
With a variable capacity compressor, however, there is the potential to eliminate the clutch, which is a fairly expensive component. Variable capacity piston compressors reduce or increase capacity by changing the piston stroke length which, in turn, is accomplished by changing the slant angle of the piston driving wobble plate relative to the rotating drive shaft. A greater slant increases stroke length, while a smaller, more nearly perpendicular angle minimizes stroke lenght. Changing the slant angle, in turn, is typically accomplished indirectly by changing the net pressure force balance seen by the front and back of the piston as the piston is pulling back within its bore. The wobble plate that drives the pistons through their stroke is pivoted and hinged to the drive shaft is such a way as to allow it to passively respond to that net pressure balance on the pistons, and to change its angle relative to the drive shaft, thereby accomodating itself to the stroke that piston follows based on the pressure balance that acts on it. This slant angle of the wobble plate changes in such a way as to keep the forwardmost or xe2x80x9ctop dead centerxe2x80x9d stroke position of the piston consistent.
The net pressure balance seen by the piston, in turn, is the difference between the suction cavity pressure, which acts on front of the piston, the crankcase pressure, which acts on the rear of the piston, behind the cylinder bores. When the piston front (suction) pressure is relatively greater than the piston rear (crankcase) pressure, the piston can retract farther in its backstroke. This pressure balance can be controlled by suitable valves that admit some of the discharge cavity pressure into, or vent it from, the crankcase, in response to suction pressure (which is a function of cooling demand), discharge pressure, or both. Such a control valve can be seen in co assigned U.S. Pat. No. 4,428,718 to Skinner, which discloses a passively acting valve. Such valves can also be directly, actively controlled, such as by an electronic solenoid mechanism, as disclosed in co assigned U.S. Pat. No. 6,038,871 to Gutierrez et al. With electronic control, there is the potential to operate the valve in response to a multitude of possible vehicle and engine parameters. Valves of this basic design are oriented in the compressor rear head, with a small diameter plunger that shifts up and down to solidly open or close various ports in the stationary valve body, so as to open or close various flow paths between and among the suction, discharge and crankcase cavities.
A different valve design is the so called spool valve, which incorporates a large diameter slidable cylindrical member or spool located at the center of the compressor housing, coaxial to the compressor drive shaft. A the spool is shifted back and forth, typically with a solenoid, various grooves and ports in the spool into or out of line with flow passages in the compressor housing. This also makes or breaks various flow path connections between and among the discharge, suction and crankcase cavities to effect the pressure balance on the pistons and the consequent piston stroke. An inherent drawback of spool valves is that a large sliding surface area between the spool and its sliding bore must be held in sufficiently close contact to provide a fluid seal. Also, the location of spool valves, concentric to the compressor drive shaft, inevitably adds a significant extra axial length to the compressor housing. Plunger type valves, by contrast, with their relatively narrow central rods, are more compact, have less mutually contacting sliding surface area, and provide a solid, on off action.
With electronically operated capacity control valves of either the spool or plunger type, there exists the potential for eliminating the clutch, since it is possible to bring the piston stroke almost to zero (by bringing the wobble plate angle nearly to ninety degrees, or no slant). However, it is not practical to bring the piston drive plate absolutely to ninety degrees, so that minimum piston stroke is a small, but still greater than zero, stroke, which causes some refrigerant pumping to occur. In low demand situations, this can potentially cause evaporator freezing, over time.
Electronically controlled variable capacity piston compressors using centrally located spool valves have dealt with the minimum stroke, evaporator freezing problem with at least two known methods. Each known method involves cutting the compressor flow off from the overall system, and providing instead an internal recirculation path within the crankcase housing to accommodate the refrigerant that the compressor continues to attempt to pump at minimum stroke.
One relatively old method, disclosed in U.S. Pat. No. 4,526,516, cuts the compressor flow through the system, at minimum stroke, by a spring biased check valve which simply shuts when the discharge pressure is low, as it is at minimum stroke. At the same time, with the solenoid deenergized, a feed back spring pulls the spool valve into a position which aligns various grooves and ports on the spool so as to establish a three way path between and among suction, crankcase and discharge to allow the pumped refrigerant to recirculate. This design requires the spring to accurately pull the spool into the position that establishes the recirculation path, a position that is dependent upon consistent spring operation, without a solid stop.
A newer design, disclosed in U.S. Pat. No. 5,584,670, provides a moving spool that cuts off flow to the suction cavity, rather than to discharge. As the spool moves to cut off suction flow, it also establishes a similar recirculation path between and among the crankcase, suction and discharge. The flow path is complex, using several dedicated passages in the compressor housing, and the overall system requires a plunger type valve in the rear head, as well as the spool, making it particularly non compact.
The subject invention provides a freeze protection feature for a clutchless, variable capacity piston compressor using a plunger type control valve, in which a refrigerant by pass path, directly from discharge to suction, is provided integrally within the valve itself.
In the preferred embodiment disclosed, the compressor housing rear head contains a plunger type capacity control valve of a known type in which a central rod or plunger is shifted up and down by a solenoid to selectively open and close a flow path from discharge, into the valve, and then to crankcase, thereby controlling the back pressure in the crankcase, to thereby control the effective piston stroke. There is, therefore, an existing discharge opening into the valve body. The disclosed valve is also the type that has a suction pressure responsive means to change the effective length of the plunger rod, and thereby change the effective opening of the discharge to crankcase flow path. This suction pressure responsive means is an evacuated bellows that resides in a chamber open to suction. While the bellows chamber is essentially static, that is, open to suction pressure, but with no substantial flow in or out, it does have an existing suction opening into the valve body.
The improvement of the invention makes use of the already existing discharge and suction ports to the valve body, and also of the pre existing motion of the plunger, to create a pumped refrigerant by pass path that acts only at minimum stroke, which is entirely integral and internal to the valve body, and which is solidly shut off at all times other than during minimum stroke operation. A secondary, passively acting, spring loaded by pass valve is provided through the valve body, between the discharge and suction ports, in parallel to the central plunger. The by pass valve is solidly shut off at all positions of the plunger corresponding to other than minimum stroke. When the plunger is fully pushed down, and minimum stroke, to fully open the discharge to crankcase flow path, it also pushes down and opens the by pass valve. A direct by pass path is thereby established between discharge and suction to re circulate the refrigerant flow an the minimum stroke position. The by pass path is inoperative at all other times. No changes to the compressor housing, and only minor changes to the valve body, are required.